1. Field of the Invention
The present invention relates generally to antennas for mobile communications, and in particular to a base station antenna used in an indoor mobile communication system.
2. Description of the Related Art
A sleeve antenna which is functionally equivalent to a half wavelength dipole antenna is described in J. D. Kraus, "Antennas", McGraw-Hill Book Company, 1988, p. 726. The sleeve antenna provides a vertically polarized radio wave with an omnidirectional radiation pattern on a horizontal plane when positioned vertically. The direction of the maximum radiation field intensity generated by the sleeve antenna is normal to an axis of the sleeve antenna. Thus, the sleeve antenna fulfills general requirements for an antenna for mobile communication.
Recently, a variety of indoor mobile communication systems using an antenna for mobile communication involving a cordless telephone, for example, have actively been developed. In such an indoor mobile communication system, communication is performed between a base station having an antenna installed indoors, and a mobile terminal.
In the case where the sleeve antenna is used for in a base station of the indoor mobile communication system, the following problems occur:
Radio waves radiated from the sleeve antenna are omnidirectional on a horizontal plane. Accordingly, when the sleeve antenna is installed adjacent and parallel to a wall in a room, for example, the antenna radiates radio waves with equal field intensities in all directions including toward the wall and into the room. Since the base station communicates with the indoor mobile terminal, e.g., a terminal within the room, there is no need for radiating the radio waves toward the wall. It is desirable to lower as much as possible the field intensity of the radio waves radiated toward the wall in order to realize efficient communication. When the base station is installed adjacent to the wall, it is ideal that the antenna be more directional and should radiate with a fan beam pattern on a horizontal plane so as to radiate the radio waves only inward into the room. The conventional sleeve antenna, which radiates radio waves with an omnidirectional pattern on a horizontal plane, does not realize efficient communication with the indoor mobile terminal.
Further, in the case where the base station includes the sleeve antenna installed on the ceiling or in an upper area of the wall, the mobile terminal is typically positioned below the base station. Since the radio waves radiated from the sleeve antenna has a highest intensity on a horizontal direction which is normal to the axis of the antenna, the radio waves radiated toward the mobile terminal is weak. This results in inferior communication.